You are now told that the function f from (a) is strictly increasing on eachinterval (4k − 1, 4k) and strictly decreasing on each interval (4k, 4k + 1).Using this, explain why f has exactly one root in each interval [4k−1, 4k]and [4k, 4k + 1].
Question
You are now told that the function f from (a) is strictly increasing on eachinterval (4k − 1, 4k) and strictly decreasing on each interval (4k, 4k + 1).Using this, explain why f has exactly one root in each interval [4k−1, 4k]and [4k, 4k + 1].
Solution
The function f is strictly increasing on each interval (4k - 1, 4k) and strictly decreasing on each interval (4k, 4k + 1). This means that for any x in the interval (4k - 1, 4k), if x1 < x2 then f(x1) < f(x2), and for any x in the interval (4k, 4k + 1), if x1 < x2 then f(x1) > f(x2).
Now, let's consider the intervals [4k - 1, 4k] and [4k, 4k + 1]. Since f is continuous, by the Intermediate Value Theorem, if f(4k - 1) < 0 and f(4k) > 0, then there must exist a number c in the interval (4k - 1, 4k) such that f(c) = 0. Similarly, if f(4k) > 0 and f(4k + 1) < 0, then there must exist a number d in the interval (4k, 4k + 1) such that f(d) = 0.
Since f is strictly increasing on the interval (4k - 1, 4k), there can be at most one root in this interval, because if there were two roots, say c1 and c2 with c1 < c2, then f(c1) = 0 and f(c2) = 0, which contradicts the fact that f is strictly increasing. Similarly, since f is strictly decreasing on the interval (4k, 4k + 1), there can be at most one root in this interval.
Therefore, f has exactly one root in each interval [4k - 1, 4k] and [4k, 4k + 1].
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